In industry several non-destructive techniques are used to determine if structural members are free of defects. For example, in the building industry, steel beams are inspected prior to use by ultrasonic and X-ray techniques to detect the possible presence of cracks. Structural members employed in the construction of aircraft are inspected for flaws both before and after their incorporation into an aircraft.
Several types of structural members are employed in the construction of aircraft. One type that is often used has a laminated structure which can be fabricated from one or more coatings or layers of a plastic material bonded with an adhesive or by a heat sealing procedure to a metal substrate. Another type of structural member has a honeycomb structure in which a layer in the form of a metallic honeycomb is sandwiched between two layers of plastic sheet material. The edges of the honeycomb layer are bonded to the sheets by means of an adhesive or by heat treatment.
As in the building industry, ultrasonic and X-ray test methods have been used to test structural members prior to their use in aircraft construction and periodically after operational use to determine if they are structurally sound. However, for several reasons such methods have not proven to be entirely satisfactory. For example, the methods involve the use of expensive and complicated equipment, requiring the services of skilled technicians. Simplified test procedures have been suggested for detecting minute surface defects that utilize dye penetrant compositions. Examples of such procedures are disclosed in U.S. Pat. Nos. 3,567,932, 3,814,695 and 3,896,664. While dye penetrants used in the prior art processes are effective in detecting surface defects, they are incapable in detecting whether a flaw penetrants to a metal substrate. In other words, the dye penetrants present the same appearance in the case of surface flaws as they do with through-the-thickness cracks. Since cracks penetrating to a metal substrate permit water and other corrosive elements to contact the substrate, it would be desirable to have an inspection method that distinguishes between mere surface defects and through-the-thickness cracks in a structural member.
It is a principal object of this invention, therefore, to provide a method for detecting flaws in composite structures comprising a metal substrate having a plastic skin or sheet bonded thereto that penetrate the skin or sheet to the substrate. Other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the accompanying disclosure.